This PPG competing renewal application is driven by the fundamental and therapeutic importance of costimulatory pathways in regulating T cell activation, tolerance, and exhaustion and builds upon our significant progress since initial award of this PPG in 2003. Synergy in our PPG is highlighted by 118 publications, 43 co- authored by multiple PPG investigators. Our PPG also has had a significant role in fostering development of junior faculty,and sharing novel mAbs and mouse strains with the broader scientific community has resulted in better understanding of costimulation above and beyond our PPG aims. Our overarching goal is to develop a comprehensive understanding of how positive and negative second signals regulate T cell activation, tolerance and exhaustion. Working together, we have discovered that the PD-1 pathway has multifaceted immunoregulatory functions. We have determined novel roles for the PD-1 pathway in regulating humoral immunity and in controlling T cell differentiation and function. We also have discovered two new coinhibitory molecules, identifying repulsion guidance molecule b (RGMb) as a PD-L2 ligand and regulator of respiratory tolerance, and Protein C receptor (PROCR) as a novel regulator of Th17 cells. These discoveries demonstrate ongoing synergy within our PPG. The sharing of unpublished results and discussion of data have inspired hypotheses and experiments bidirectionally in all PPG projects, which drive the focus of our renewal application. Our major goals are to investigate: 1) Roles of PD-1, PD-L1 and PD-L2 in regulating protective and pathogenic humoral immune responses, 2) Roles of the PD-1 pathway in regulating the balance between protective and pathogenic subsets of T cells, 3) Function of RGMb in controlling T cell activation, tolerance and exhaustion, and 4) Relationships between PD-1 and PROCR in controlling Th17 cells and exhausted CD8 T cells, using models of infection, chronic graft versus host disease (cGVHD) and autoimmunity. Our proposed Program will consist of 3 highly integrated and interactive Projects, supported by 3 Cores. Project 1 (Sharpe/Ahmed) will focus on the roles of PD-1, PD-L1, PD-L2, RGMb and PROCR in controlling protective immunity during acute and chronic viral infections. Project 2 (Blazar/Turka) will investigate how PD-1 pathway members, PROCR and CD28 regulate cGVHD. Project 3 (Kuchroo/ Sharpe) will study how PROCR and PD-1 influence pathogenic vs. non-pathogenic Th17 cells and thus regulate autoimmune diseases. Administrative Core A (Sharpe) will provide administrative and scientific coordination. Antibody/Ig fusion protein Core B (Freeman) will provide novel mAbs and Ig fusion proteins. Transgenic/Knockout Core C (Sharpe) will provide a unique and important set of mouse strains. The use of the same standardized tools makes it possible to compare and contrast results in different settings and disease models. Mechanistic insights from our proposed studies should guide translation into new immunotherapies targeting costimulatory and coinhibitory pathways for human chronic viral infections, cancer, autoimmune diseases and transplantation.